The present invention concerns a device and a method for assessing the structural characteristics of supporting poles installed or sunk in the ground, particularly supporting poles made of wood, reinforced concrete, fiberglass and the like.
In many rural areas, links with telephone and/or electrical networks (lines) are of the “overhead” type, i.e. telephone and/or electrical cables are carried on top of substantially vertical and sunk in the ground supporting poles. Said supporting poles are, usually slightly tapered upwards, present generally circular in cross-section and are typically made of wood.
It is often necessary to carry out maintenance work along a line. Such maintenance work can be planned over time or can be required occasionally, e.g. after an unexpected failure of the electrical and/or telephone cables supported by poles. Line maintenance work requires the intervention of operators who, by using ladders or crampons, climb to the top of the supporting poles to carry out the required operations.
Supporting poles are originally sunk deeply in the ground and must maintain over time structural characteristics sufficient to allow an operator to climb up to the top of a pole safely, without risking pole being broken or overturned, e.g. when its portion sunk in the ground is not sufficiently firm. Fatal accidents due to a supporting pole in a line being broken during maintenance work are unfortunately anything but uncommon.
In order to avoid serious accidents to operators and faults in line cables, periodic inspection of the structural conditions of poles is required. Should poles not meet pre-determined technical requirements, they must be replaced.
In the present description and in the claims, the term “constraint conditions” of a supporting pole is used to describe a set of forces exerted on the pole, in order to keep it in an substantially vertical position, also on steep and impervious ground. Constraint conditions, therefore, include both the action exerted on the pole by the terrain in which it is sunk (the so called ground-level constraint), and forces applied to the pole by any auxiliary supporting pole and/or by tension wires suitably applied to it (the so called ground anchoring system).
In the present description and in the claims, the term “material degradation” of a supporting pole is to be understood to mean degradation of mechanical characteristic features of the material of which the pole is made of, typically wood, i.e. modulus of elasticity, mechanical resistance and volume mass, and possibly reduction in cross-section of the supporting pole actually capable of withstanding stresses caused, for example, by an operator climbing to the top of the pole.
The term “structural degradation” of a supporting pole means, instead, any modification in the static conditions of the pole with respect to its initial installation, e.g. due to modifications in constraint conditions, and more particularly modifications in the ground-level constraint or in the geometric characteristics of the installed supporting pole, especially those modifications which result in a reduction of the pole resisting cross-section.
That being said, factors which can compromise the reliability of installed wooden poles, e.g. in telephone lines, are essentially two: “material degradation”, i.e. the degradation of the wood forming the pole, and “structural degradation” of the pole.
“Material degradation” of a wooden pole occurs with a progressive decay of the wood, e.g. due to attack by fungi. Such a decay is generally limited to the underground portion of a supporting pole and can also affect supporting poles perfectly integral and sound above-ground.
In so far as the “structural degradation” is concerned, modifications in the constraint conditions or in the initial geometric conditions can occur, for instance, owing to incorrect installation of the supporting pole, or damage to the ground anchoring system (e.g., breaking of an auxiliary supporting pole), collapse of the ground in which the pole is sunk, or organic degradation of the underground portion of the pole.
All the above mentioned degradation factors can obviously impair the safety of a supporting pole. Thus, it is essential to assess the risk an operator, who is going to climb to the top of a supporting pole to carry out maintenance work, is running. Such an assessment should be in “closed form”, i.e. it should make it possible to spot the existence of risk for the operator and the gravity of such risk, independently of the cause or the specific type of degradation of the pole being assessed.
There are currently available several (manual or automated) methods for assessing the structural characteristics of a supporting pole.
Manual methods of assessment have long been disclosed, which require shaking a supporting pole being tested in a direction perpendicular to both the longitudinal axis of the pole and the direction in which the cables supported by it extend, in order to ascertain whether the pole is suitably sunk in the ground. is The pole is, then, struck with a hammer and to the sound emitted by the poles listened to: a dull and hollow sound indicates a poor state of the pole structure. Then, by digging the soil at the base of the pole to a depth of about 30 cm, i.e. in the area where pole can more easily decay, the state of preservation of the wood is visually ascertained.
Other methods involve, instead, the use of automatic devices. The method developed by the Swiss Federal Institute of Technology in Lausanne provides for the of e.g. a device, termed POLUX that by means of two suitably shaped electrodes which are inserted into the supporting pole being assessed at ground level measures the force of penetration exerted by the device while the two electrodes are inserted and the degree of humidity of the pole wood. The force of penetration is correlated with wood density, and thus with the resistance of the supporting pole to bending stress applied to the pole, e.g. by the operator while climbing the pole, whereas the degree of humidity is correlated with the extent of biological decay of wood fibers.
According to another well-known method, a device, commercially known under the name of RESISTOGRAPH® (distributed by RINNTECH of Heidelberg, Germany), is employed which is suitable for recording the wood resistance to penetration of a suitably shaped drill bit, said drill bit rotating and advancing in the wood at a constant speed. The wood resistance to the drill bit penetration is correlated with structural characteristics of the pole.
Another known method requires the use of a device termed POLESCAN (manufactured by IAMSL Ltd., New Zealand), which makes it possible to test a pole by means of ultrasound probes positioned at the base of the supporting pole being assessed.
Such manual procedures have the disadvantage of being subjective, since assessment of pole characteristics depends on subjective evaluation by the operator. Bearing in mind that in order to carry out maintenance operations of a line an operator has to climb the poles, understandably enough often he is willing to be strict in his assessment, and as a consequence about 20% of the poles classified as “to be replaced” are still in good condition, i.e. having structural characteristics meeting pre-established technical requirements of a high degree of safety.
In so far as the methods employing the above mentioned automatic devices are concerned, they have the disadvantage of relying on measurements which, although objective, regard exclusively the assessment of the wood conditions, and do not consider the integrity of the geometric characteristics or the ground-level constraint of the pole.
In addition, as these methods are based only on local examinations, they measure the structural properties of a wooden pole only at a pole point or area where these measurements are made, and thus they are not representative of the whole structure of the supporting pole.
Another drawback of automatic methods is that they use relatively large devices, too heavy and cumbersome for an operator who has to move along the line, between one pole and the next, for example in wooded or cultivated country.
Another drawback of the methods known in the art is that they do not provide any evidence of an executed test. In case of fault along a line or an accident to the operator, it is, indeed, desirable and advantageous for the company responsible for the maintenance along that line to be able to prove that the assessment and planned maintenance of the poles in the line was carried out in an accurate and punctual way.
Not the last drawback of known methods, both manual ones and those involving the use of devices such as RESISTOGRAPH® and POLUX, is that they are invasive, with the risk of compromising the structural characteristics of a pole being measured, as they require, e.g. removal of a sample of pole material, or drilling holes in the supporting pole.